He looks down in the tooth

This is a mash-up of “down in the mouth” (gloomy, depressed) and “long in the tooth” (old, past his/her prime). The confusion here is pretty clear: teeth are in the mouth and the words “down” and “long” are four letter words. What I can’t recall is whether the speaker was referring to a person who seemed older or more depressed. Or maybe both as they go hand in hand??